Russia struck an arms event near Kiev on July 24, killing multiple people and wounding dozens, Ukrainian prosecutors and defence industry representatives said.
Ukraine's prosecutor general reported at least 10 dead and around 100 wounded in the missile strike.
He did not say whether all the victims were at the gathering.
"An investigation will establish who made the decision to hold the event... and whether the risks were properly assessed under martial law," prosecutor general Ruslan Kravchenko said, announcing a criminal inquiry.
"A rescue operation is currently underway in the Kiev region following a Russian missile strike," Ukrainian President Volodymyr Zelensky said, without elaborating on what had been hit.
Andriy Grytseniuk, head of the state-backed defence group Brave1, described the strike as a "horrific tragedy".
"The enemy launched a ballistic strike on an event organised by a private association of manufacturers," he said.
"I strongly urge everyone involved in the defence industry to urgently review and reinforce their safety protocols," he added.
Kiev does not typically announce information about Russian strikes on military targets.
But there was a backlash from many in the industry and officials, who implied that it had been poorly organised and demanded accountability.
The event had been advertised online in advance, according to Ukrainian media.
Russia has repeatedly targeted Ukrainian training grounds and military events throughout the war.
